Remote areas suitable for the development of wind power plants, complex terrains and the turbulent nature of the local winds are affecting the installation of wind turbines. Therefore, the introduction of new and innovative wind energy systems that produce high output regardless of the wind speed and patterns are desired.
It is imperative to understand better various ways of improving the efficiency of wind turbines, as well as bringing down the cost to an economical level. The cost of oil and other non-renewable resources are on the rise every day (Carroll, 2014). Improving the efficiency of wind turbines helps in reducing the energy limits being experienced today. In the future, turbines will be able to harness more energy from the wind with limited technological improvements.
A considerable number of high-efficiency turbines lowers the cost of energy as well as generate power at a lower cost. The wind shroud system is one of the major techniques that can be utilized to improve the efficiency of the turbines. The wind shroud systems have a duct that surrounds the blades that increase the cross-sectional area of flowing wind. The resultant sub-atmospheric pressure within the duct draws more air through the turbine blades and hence providing more power to be generated compared to the traditional turbines (Bet & Grassman, 2013).
The generation of wind power is proportional to the speed of wind cubed(Phillips, 2003). Therefore, an increase in output is caused by the possibility to increase the speed of the wind and using the wind fluid dynamic around the structure, the output of power of the wind turbines can be increased considerably. Even though there have been some studies on shrouded wind systems reported, it has not provided attractive subjects conventionally. Specific research conducted extensively is the examination of shrouded wind turbines by Çetin, Yurdusev, Ata, & Ozdamar (2006).
In most of the recent studies, the focus concentrated on the techniques used to concentrate the wind in the duct with a wide-open angle, and a boundary segment controlled with a number of flow slots is used in order to concentrate the flow that goes inside the surface of the duct. Therefore, the technique of using boundary segments reduces the loss of pressure through flow separation and increasing the mass flow within the conduit.
Based on this concept, a company in New Zealand developed the Vortex 7 diffuser augmentation turbine (Abe, et al., 2006). They utilized a slotted duct that would help prevent separation within the duct. Additionally, they built a ring profiled structure in the wide opening of the turbine. It was reported that the newly modified structure increased Power output due to the wing system by a factor of 1.8 compared to the bare turbines (Abe, et al., 2006). Even though there have been, other ideas being reported, most of them are yet to reach commercialization.
The primary objective of the present study concerning developing a wind system with higher output in determining the best way to harness wind energy in a more efficient way as well as the right shroud that can be used to generate energy more efficiently. As an alternative, the shrouded wind system technology might provide energy to areas with turbulent wind patterns as well as other remote areas, where electricity is yet to be installed. In fact, shrouded wind systems are an attractive alternative for the developing market especially for areas that lack electricity or areas with energy deficiency.
Shrouded wind systems have many advantages over the bare turbines. The newly innovated turbines are easy to install since they have a shorter construction period that is extending the utility grid line. This turbine technology has been reported to be lesser complex to construct (Hirahara, Hossain, Kawahashi & Nonomura, 2005). As for the reasons, local manufacturing is usually a preferred option in remote areas or developing areas that could, in turn, boost economic development as well as reducing the cost of production.
This model of turbine technology would also increase the wind velocity through the blades which include a small inlet shroud and a diffuser. A low-pressure area is usually generated behind the diffuser as vortices are created. Not only there is a huge development in the wind industry in regard to power production through the wind turbines, but also on the effectiveness of the turbines used. Recent research shows that the researcher is looking for much better ways of improving and increasing the effectiveness and applicability of the present wind turbines (Hirahara, et al., 2006).
As it is being observed that the wind is the most natural and occurring source in order to produce energy. There is excessive importunity for energy because of the industrialization and the rapid growth of the economy. Further, there is a great maximization in fossil fuel consumption in order to generate energy (?uriši?, Ž., & Mikulovi?, J. 2012). In the year 2013, the content of global carbon dioxide increased from 280 ppm to 400 ppm in the environment. The recent report which is given by the NOAA (National Atmospheric and Oceanic Administration) in the year 2016, the content of the global carbon dioxide was maximized to 402.59 ppm from the 399.29 ppm in the environment.
The negative impact of the fossil fuels discharging carbon dioxide and the various other harmful gases, perpetual maximization in the cost of fossil fuels and the huge consumption which further leads to abatement of resources for the upcoming generations and various parts of the world has demonstrated great interest in the technologies of wind energy in order to produce power. Further, the wind turbine’s power output relies highly on the velocity of the wind. It is responsible for the blades rotation to transform it into electrical energy from the mechanical energy. Moreover, the velocity of the wind is amused towards the turbines of the wind in order to produce the power at maximum level.
Further, the usage of wind energy was started 3000 years ago, and it was initially accustomed in order to grind grain through the windmills drive to pump water and shove ships. There are two types of energy sources which are available in order to produce the power i.e. conventional and non-conventional. Wind energy is the component which is non-polluting and eco-friendly and further it is a technique which is being used in order to generate power, and it doesn't emanate carbon dioxide and various other harmful gases in comparison to fossil fuels and the generation of nuclear power.
The wind turbines are the most effective sources which are producing power by transforming kinetic energy to the mechanical energy, and further, it informs to the electrical energy through the generator. Furthermore, adjudicating the placement of the turbines of the wind in the wind far, accustomed to contain simplified models and the guess work but the numerical techniques are empowering optimization and the understanding of the turbine's wake interactions (Kubik et al., 2013). Without the usage of these techniques, the wind energy would not be exploited to its potential. ABL (Atmospheric Boundary Layer) was also the concern of various speculations but the dimensions coupled with the new techniques of numeric which have helped in making the accurate measurements of wind energy of the site according to its potential. In these areas, the advances have been made, but it still requires more improvisation and improvisation can be done through optimization and combination of the present contrived models.
The numeric methods and the dynamics of the computational fluids can be utilized in order to anatomize the flow over the complicated terrain to better micro-site the farm of wind. The coastal areas are the most fascinated locations due to moderately high speeds of the wind but these regions mostly have an unsteady flow of the wind and the complicated terrain. The dynamics of the computational fluids can be coupled along with the algorithms that use spacing, cost, inter-connections of the turbine and geography in the form of variables to find the great layout of the farm. Further, the single kind of the turbine at height can be anatomized, and the various other restrictions and the price are not being considered (McWilliam et al., 2012).
The impact of support structures on the flow of wind might have an effect on the performance of the turbine. The impacts can be caused by using the characteristics of the flow, interference of the blade tower, lift forces by using the dynamics of the computational fluids on the upwind turbine. Further, this model predicted successfully the flow deterrent from the tower when the blade will pass through it. Despite this, it doesn't demonstrate that the outcomes were substantiated by experimental data. Moreover, the interference of tower gives rise to vibrations. The simulation of numeric and the anatomy of the wind turbine structures of the support and the generation of noise are the main areas to study because they are accustomed to augur the fatigue life and minimise a load of fatigue towards the support structure of the turbines of wind.
The wind turbines can be characterised in two kinds relied on the axis in which the rotation of the turbines takes place i.e. wind turbine of the horizontal axis and the wind turbines of the vertical axis. Further, in the horizontal axis, the turbine blades rotate around the axis of horizontal, and they are mainly pointed to the direction of the wind. Whereas, the wind turbines of the vertical axis are the kind of turbine of the wind in which it rotates vertically. The wind turbines of the vertical axis are the independent of the direction of the wind and further, there is no need of creating a control system for the dictate adjustment of the blades, and the turbine can be accustomed in various places where the direction of the wind is largely variable. The gearboxes and the generators can be put near to the ground, and as a result, there will be a minimum load on the tower and the maintenance will be easy.
The main objective behind the development of the wind turbine is maximizing the power outcome of the turbine. Moreover, there are two parameters that impact the power's value i.e. the blade's swept area and the speed of the wind. The power outcome can be maximized by maximizing one out of the two given parameters. As a result, the conventional turbines of the wind the coefficient of the power is a restricted parameter. Moreover, the method to maximize the velocity of the wind is to utilize the duct across the rotor (Song et al., 2012). The design is being pertained as the wind turbine of diffuser augmented. It is being observed that the duct across the rotor maximizes the air's flow rate through the swept area with the help of the rotor and maximizes the velocity of the wind at the rotor. The effective pasture of the speed of the wind for producing the urged power which is broader than that connate to the conventional system. The majority of the development of the diffuser was don on the basis of the wind turbine of a horizontal axis, and it is being observed that the design of the diffusers was mainly focused on the applications of the wind turbines of the vertical axis.
A simulation result conducted by Bet and Grossman (2003) has reported a substantial increase in coverage of low-pressure areas behind the turbine. Along with the studies above, this section will present the main theories for the assessment of the shrouded wind turbine as compared to the bare turbine for turbine aerodynamic evaluation. A preliminary review of the literature is made to understand the fundamental concepts of the Shrouded wind turbines. Some of the key contributions made by researchers in the field in the past few years will be discussed.
The invention of accurate models of aerodynamic aspects of wind turbines is one significant way of analyzing different wind energy systems. The operation of wind turbines induces phenomena like wind flow components where the direction and the magnitude of the wind are the rotor changes and the turbine blades rotate (Cetin, Yurudusev, Ata & Ozdamar, 2005). Furthermore, wind flow separation becomes more complicated in the new model. The instabilities of the wind interacting with the blades, hub and the tip of the vortices impact the character of the entire flow. The aerodynamics of the wind turbines becomes more complex with all the wind instabilities as well as flow interaction. (Burton, Sharpe, Jenkins & Bossanyi, 2001).
First, in order to understand the complex physics and aerodynamic used in developing the shrouded wind turbines one must analyze and understand a simple one-dimensional model (Archer & Caldeira, 2009). To understand the concept of the shrouded rotor, it is also important to examine the theoretical concept which governs the performance of shrouded wind turbines.
According to the Augmentation ratio era, it is easy to describe the enhanced energy produced by the turbines combined by the diffuser. Thus, the augmentation ratio is expressed as (Aranake, Lakshminarayan & Duraisamy, 2013): ra= (Cp, d)/0.593, these Cp,d represents the power coefficient for a turbine with the diffuser.
Shrouded Wind turbines can combine various systems to assist in concentrating and accelerating the wind. A hollow structure is used for surrounding the turbine as a way of enhancing the flow of wind. Applying a nozzle in a converging shape at the inlet of the shrouded wind system is beneficial especially in turbulent wind flow condition, which is common in urban areas (Hirahara, et al., 2005).
The shrouded turbines exploit the venturi effect, where a reduction in pressure of the wind and the velocity of wind are produced by the passage of the fluid in a contraction. For a shrouded wind system, contraction is performed by the shroud surrounding the turbines (Isensee, et al., 2012). At the nozzle section, the diameter of the inlet should be larger than the outlet in order to lower the wind velocity. Also, in regards to the outlet, the velocity is increased due to a decrease in the area (Vermillion, Grunnagle, Lim & Kolmanovsky, 2014).
Albert Betz, a German physicist, calculated that no wind turbine could convert more than 59.3% of the kinetic energy of the wind into mechanical energy turning a rotor. This is known as the Betz Limit and is the theoretical maximum coefficient of power for any wind turbine. In bare wind turbines operating at a maximum Betz limit, the flow of air decreases to approximately 2/3 of the velocity of the free stream. This decrease in airflow causes an increase in pressure in front of the rotor inducing a small section of the mass flow being pushed around the rotor (Khunthongjan & Janyalertadun, 2012).
A way of increasing the flow of air can be incorporated by using an annular lifting device around the turbine. This device is what is known as a shroud. The increase in the exit plane of the shroud together with a reduction in the exit pressure enhances the mass flow leading to higher extraction of energy.
The philosophy behind a flange at the exit of the shroud diffuser is to create a low-pressure system at the downstream to increase the velocity of incoming air. Past research conducted on the flange diffuser only focused on the impact of the width of the vertical Fringe (Carroll, 2014).
Grady & Hader (2012) developed a diffuser to a bare wind turbine so as to increase the efficiency of the wind turbine and also conducted an analysis of the fluid flow. The outcomes of the simulation indicate that there was an increase in velocity of the wind by approximately 60% (Isensee, et al., 2012).
Toya, Kusakabe & Yuji (2007) developed a convergent and a divergent shroud wind system using an any. Later analysis of the wind flow was conducted on the turbine with a shroud and a bare turbine. The outcome showed that the output of the turbine with a shroud increases three times compared with the one with no turbine (Toya, et al., 2007).
In the recent study, it has been observed that the researchers are making the use of a conical shroud in the tunnel of the wind because the member of the conical is having a frequently maximizing the diameter in the direction of the wind flow. Also, the member of the conical expands smoothly in the direction of the wind flow. The efficacy of the conventional turbine of the wind is restricted. The system of the wind turbine which consists of shroud along with a cone was created in order to face these challenges.
According to the researchers, global warming is described as one of the greatest threats that everyone must be aware and should act responsibly in controlling and managing the causes of global warming. It has been researching that approximately 80 percent of global warming is caused by the emission of carbon dioxide through fossil fuels. Further, wind power has the capacity the control the disaster of the environment because of it consumers around 0 percent of the water and it is environmentally friendly, and it largely reduces the emissions of the carbon dioxide content (Wan et al., 2012).
The researchers have stated that the capacity of the power of the wind turbine mainly relies upon the design and the speed of the wind. Moreover, it has been observed that the only disadvantage of the technology of wind turbines is that the low speed of the wind possesses minimum energy of the density per volume of the air which hits the blades of the turbine and maximizes the cost of production in comparison to the fossil fuels. The researchers have conducted various researches in order to improvise the density of the energy in the wind as it will further improve the capacity of the power and would make the resources cost-effective.
According to the researchers, predicting and modeling the attributes of the wind are significant to the power sitting and the prediction. Further, the accurate models of wind prediction exist but the methods of numeric improvise the outcome. The example is the technique of predicting the potential of the wind power through a program which is called WAsP (wind analysis of the Atlas and the application program). Further, the program uses the velocity of the wind and the direction at the individual elevation. Moreover, the method so the minimum squares are being used in order to convert the wind data which is being taken at three or even more elevations to the data of equivalence which is being taken at one so that the accurate data can be used as the WAsP’s input.
Some of the researchers (Saha, U. K., & Rajkumar, M. J. 2006) have also used the simulation models of ABL in order to appraise the wind farms' locations. Further, the simulation model of MM5 ABL was used to impose the potential area of China’s coastal area. The model estimated the distribution of the velocity of the wind and the energy accurately in the ABL. The connate study was being used as the model of WRF in order to stimulate the wind with the wind farm of the country through the complicated terrain. The model has the distinct boundary layer and the schemes are proceeding for the distinct levels of the terrain and the types of the vegetation.
The researchers had planned to run the model of the CFD coupling and the wind prediction through two distinct resolutions and further validate it through the various observations. Therefore, the methodology was proved to be accurate apart from such cases where there was the existence of the sharp and abrupt transformations in the terrain like cliffs and hills.
Some of the researchers (Akhgari, A. 2011) have also focused on the analysis of the fatigue by using the techniques of numeric. Further, the model of FEA is being used in order to identify the blade's critical zone and accelerated the fatigue defect modeling to predict the fatigue life of the blades of the wind turbine. Furthermore, the wind turbines of the horizontal axis are mainly used to produce the power, and there are various experimental and numeric studies which are related to the applications of the wind turbines of the horizontal axis. Further, the researchers have studied the wake architecture of the turbines in a tunnel of water by using the turbine of a small scale. The researchers have made comparisons on the basis of the experimental outcomes through the calculation of the numeric depending on the method of rotor vortex lattice and the code of the custom.
The researchers (Fujisawa, N., & Shibuya, S. 2001) also authenticated the experiment through the experiment of the full scale in the tunnel of the wind and moreover the flow architecture of the turbines with the diffuser has been studied. The technique of PIV is being used in order to illustrate the structure behind the diffuser and also in the rotor’s cross-section. Further, the researchers used the design of the diffuser and also established the blockage vertices formation delinquent the diffuser because of the pressure drop.
According to the researchers (Islam et al., 2008), the experimental study was conducted in order to analyze the performance of the turbine of the diffuser augmented in the visualization flow of the water tunnel. It is the experiment which is being done in order to analyze the hydrodynamic behavior of the sunken bodies in the flow of the water. The tunnel of the water is comparable to the re-circulating tunnel of the wind apart from the working fluid which is water, not the air. Further, in accordance with the adequate scaling, the experiments can be done through the turbine model of the small scale. Another difference that is being observed between the air and water tunnel is the passage that the driving force is being produced in each system. Therefore, the power outcome of the turbine when there is the presence of the diffuser across the rotor which is lower as compared to the power outcome of the bare rotor for the ratios of the low speed.
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